1. Field of the Invention
The present invention relates to an organic light emitting devices and other devices incorporating the same.
2. Description of Related Art
Light emitting devices are well known and have been used in many applications. In simple terms electric signals are used to stimulate light emissions and the emission of light from organic materials is associated with electronic transitions between energy levels.
An example of a basic device is shown in FIG. 1. The devices are normally structured as two electrodes (2,4) sandwiching a photoemitting layer (6) and provided on a transparent substrate (8). The electrodes are used to cause the movement of electrons and holes into the photoemitting layer and the recombination of the electron/hole pairs in the photoemitting layer results in light emissions.
Typically, the transparent substrate (8) is formed from glass, silica or a plastic. The upper, cathode electrode (2) is typically formed from Ca/Al or Li/Al or LiF/Al. The lower electrode (4) must be transparent and is typically formed from ITO (Indium Tin Oxide), ZnO2, Indium Zinc Oxide or GaN.
The photoemitting layer (6) may preferably be formed from an organic material. The organic material may be of a low molecular type as described in the Article by Q W Tang in Applied Physics Letters 1987 pages 913 to 915. Alternatively, the organic layer may be of a polymer type as described in the Article by J H Burroughes in Nature 347 on pages 539 onwards published in 1990. Examples of low molecular type materials are shown in FIG. 2. Examples of polymers include polyfluorene and some of the derivatives thereof and these are shown in FIG. 3.
Light emitting devices using an organic layer (OLED) can be used in colour displays since the colour of the emitted light can be predetermined by selecting the particular chemical structure of the organic layer.
One of the advantages of organic materials is that they are relatively easy to handle during manufacture. For example, low molecular types may be applied by sublimation, e.g. vaporisation, whereas polymers may be applied by spin coating. During the subsequent processing of the device, a number of further features will need to be patterned. Such patterning using photo lithography often degrades the light emitting efficiency of the device. The impact of patterning can be reduced when using organic materials since low molecular types enables metal masks to be used and polymer types enables ink jet deposition techniques to be used. The ease of manufacturing the organic material itself and the improved resulting performance of the device following subsequent patterning, also enables large size devices to be formed.
As discussed previously, LEDs function by stimulating the injection of electrons and holes into the photoemitting layer. FIG. 4 illustrates in more detail the injection of electrons and holes and the recombination of electron/hole pairs, resulting in the generation of light emissions. FIG. 4 illustrates in 1) charge being injected, 2) charge being transported, 3) charges being captured to form an exciton and 4) electron/hole pairs recombining. In order to maximise the light emissions, the energy levels between the materials of the electrodes (2, 4) and the photoemitting layer (6) need to be carefully selected. The energy levels need to be selected such that there is an appropriate energy level gradient from the transparent electrode (4) to the cathode (2).
To improve the efficiency of OLEDs conjugated polymers may be used instead of a single layer of organic material. Conjugated polymers provide an interface within the photoemitting layer such that electron/hole pair recombinations tend to become concentrated at the interface. This leads to a concentration of light emissions from the immediate vicinity of the interface and reduces the possibility of direct transmission of electrons or holes across the photoemitting layer. The polymers shown in FIG. 3 are particularly suited to use in a conjugated polymer organic layer in an LED.
Waveguides enable light to be transmitted more efficiently from a source to a desired point of application.
Waveguides also form an essential part of the structure of semiconductor lasers, an example of which is shown schematically in FIG. 5. The arrangement comprises an inorganic photoemitting layer (12) disposed in a confinement layer (11). On each side of the confinement layer is a cladding layer (10), each of which is supported by a substrate (2). Light propogates in the photoemitting layer (12) and the confinement layer (11) and is reflected by the mirrors (14) (one of which is semi-transparent) such that a narrow, highly concentrated beam of coherent light is output by the laser.
In the semiconductor laser, the electrodes are not of course made of transparent materials.
Faced with a desire to concentrate the light output from an OLED, it would seem natural to consider the use of a waveguide. In addition, the structure of semiconductor lasers would lead to consideration of an OLED in which both electrodes are optically opaque and in which a waveguide is arranged to be xe2x80x9cin-planexe2x80x9d with the light emitting layer. However, a number of difficulties arise. Importantly, organic materials tend to have a low conductivity. Therefore, in practical devices, the layer needs to be relatively thin, often in the order of 200 nm for many of the polymers. If the photoemitting layer is this thin, the output of a corresponding in-plane waveguide would exhibit a diffraction effect which would lead to poor coupling efficiency of the device with other optical devices. If the thickness of the layer is increased, then due to the poor conductivity of the organic material, the driving voltage tends to become prohibitively high.
Against this background, the present invention provides an organic light emitting device comprising: a first electrode and a transparent electrode with an organic light emitting layer therebetween; characterised by comprising a waveguide provided on the opposite side of the transparent electrode compared with the organic light emitting layer.